Modern communication systems are more commonly using linear modulation techniques due to the high spectral efficiency these techniques provide. The complexity of the modulator and tolerances required often require formulation of the RF signal in its baseband equivalent by a digital signal processor. Hence, baseband to RF conversion is required. Because linear modulation is employed, a linear power amplifier (PA) is required. A well known method for creating a linear RF PA is to employ feedback. Because the input to the transmitter is a baseband signal, it is convenient to down convert the RF signal to baseband with the same local oscillator as the up mixer in order to complete the feedback loop. A simplified version of such a system is shown in FIG. 1.
DC offsets are inherent, undesirable outputs or inputs of active analog active circuitry. Offsets occur in amplifiers from the inability to perfectly match components. The problem is compounded by component variation over temperature.
FIG. 1 illustrates various DC offsets that are inherently present in a baseband feedback transmitter. DC offsets result in a spectral term at the frequency of the local oscillator at the output of the system, and are often referred to as carrier feedthrough. Also shown in its equivalent form is direct local oscillator leakage, bcos (.omega.t+.phi.), into the RF path. This leakage can result from RF coupling from the mixer's local oscillator input to the mixer's output and is represented by the local oscillator signal reduced in amplitude by a factor b and phase shifted by a term .phi.. If the modulation system is designed to be a carrier suppressed system, carrier feedthrough of a sufficient magnitude will result in system performance degradation. DC offsets are typically temperature dependent, hence, adaptive means are often used to help reduce them. FIG. 1 shows three offsets, V.sub.1, V.sub.2, and V.sub.m. V.sub.1 represents an equivalent DC offset for offsets from the downmixer, the amplifier with gain B, and offsets appearing at the input, X, of the main summer 101. V.sub.2 represents the DC offset of the amplifier with gain A .sub.1. V.sub.m represents the DC offset of the upmixer. DC offsets are usually not accessible at the output of the equivalent summer, however, they can be observed at the output of the amplifier the summer is connected with.
A spectral term at the frequency of the local oscillator at the output of the system can cause a number of problems, including increased splatter at turn-on or turn-off for Time Division Multiple Access (TDMA) systems, degraded performance from the demodulated signal, and degraded efficiency of the PA due to an extra signal. The effects of the DC offset are especially a problem for a zero-IF (Intermediate Frequency) transmitter, because the carrier is inband and filtering cannot generally be used to eliminate the carrier. Adding a fixed amount of DC to reduce carrier feedthrough is well known.
Accordingly, there is a need for a linear zero-IF transmitter with DC offset reduction such that carrier feedthrough is minimized.